A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a lithographic patterning device, which is alternatively referred to as a “mask” or “reticle,” may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (i.e., resist).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, while in so-called scanners, each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The term “patterning device” used herein should be broadly interpreted as referring to a device that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit (IC).
The patterning device may be transmissive or reflective. Examples of patterning means include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
Lithographic apparatuses generally employ a motion control system. The motion control system comprises a position detector for detecting a position of the substrate table in at least a plane, i.e. in at least two dimensions and a controller constructed for driving the actuator in dependency on an output signal provided by the position detector. The motion control system thus ensures that the substrate table is in a correct position (within a certain tolerance range) as a position of the substrate table is detected by the position detector, and a difference between the detected position and the desired position is reduced by an appropriate action of the controller. The position detector and controller thus form part of a feed forward and/or feed back control system.
In a current lithographic apparatus, a desired accuracy for the substrate table (also sometimes called a wafer table or wafer stage) is in an order of magnitude of nanometers. Hence, it is required according to the state of the art that the position detector achieves such high accuracy. Furthermore, requirements on the position detector are also high in that the range within which the position detector is required to operate encompasses a range of movement of around 0.5 m, as the substrate table in a lithographic apparatus according to the state of the art is able to make movements in two dimensions, i.e. in a plane covering around 0.5 m×0.5 m. To achieve these requirements, according to the state of the art, the position detector comprises one or more interferometers, preferably an interferometer for a first dimension and an interferometer for a second dimension perpendicular to the first dimension. A disadvantage of the interferometers however is that it is an expensive position detector.
A further type of position detector, well-known in the general state of the art, is an optical encoder. The encoder consists of a light source, a grating and a detector. By moving the grating with respect to the light source and the detector, changes occur in the light pattern as received by the detector due to e.g. reflection or transmission changes. The grating is thus comprised in an optical path from the light source to the detector and by movement of the grating, the pattern as received by the detector changes. From these changes, displacement of the grating with respect to the light source and detector can be calculated. From these displacements and knowing a starting position, a position can be calculated. As will be known to the skilled person, the above describes an incremental encoder, the skilled person will be familiar with the fact that also absolute encoders exist.
A specific type of optical encoder has been described in co-pending US Patent Pub. Application 2002/0041380, which is incorporated herein by reference. This optical encoder comprises a diffraction type encoder comprising a light beam generator constructed for generating a light beam, a first grating, a second grating, the second grating being movable with respect to the first grating, and a detector arranged for detecting a diffracted beam of the light beam as diffracted on the first and the second grating, one of the gratings being mechanically connected to the substrate table, the other one of the gratings being mechanically connected to a reference base of the lithographic apparatus, a movement of the substrate table causing a movement of the first grating with respect to the second grating and in operation causing a change in the diffracted beam.
The known lithographic apparatus comprises a motion control system for controlling a movement of the substrate table. The substrate table is movable in at least two directions under control of the motion control system. The movement of the substrate table is to be understood as a movement of the substrate table with respect to the projection system, i.e. a movement of the substrate table results in a movement of the patterned radiation beam with respect to the substrate.